MOSSBAUER-SPECTROSCOPY OF QUENCHED HIGH-PRESSURE PHASES - INVESTIGATING THE EARTHS INTERIOR

New phases formed at high pressure and temperature can be successfully quenched to ambient conditions if the kinetics of the back transforma tion are slow. One important application of this technique is to miner al physics - the combination of microscopic measurements of the struct ural, physical and chemical properties of high-pressure minerals measu red in the laboratory with macroscopic geophysical and geochemical dat a to produce a unified description of the Earth's interior. The domina nt high-pressure phases inferred to be present in the Earth's mantle h ave been synthesised using a multi-anvil press to correlate structural and chemical information obtained from Mossbauer data with bulk geoph ysical and geochemical data to determine properties of the mantle, suc h as the oxidation state. Results from experiments on the major transi tion zone minerals beta-(Mg, Fe)(2)SiO4, (Mg, Fe)SiO3 garnet and gamma - (Mg, Fe)aSi04 spinel include the discovery of significant Fe3+ in th ese phases synthesised in equilibrium with metallic iron and excess si lica, implying that the oxygen fugacity (fO(2)) of the transition zone must be substantially lower than the upper mantle fO(2). Mossbauer sp ectra of the lower mantle phase (Mg,Fe)SiO3 perovskite show that Fe2occupies the large distorted site almost exclusively, and that signifi cant Fe3+ is present at the minimum fO(2) stability limit. Low tempera ture spectra indicate a phase transition possibly corresponding to a d istortion of the perovskite structure. Mossbauer spectra of FexO quenc hed from high pressure indicate that the Fe3+ content of samples in eq uilibrium with metallic iron is small, implying that the minimum amoun t of Fe3+ in the lower mantle (Mg, Fe)O at lower mantle conditions is also small. Mossbauer spectra from samples of (Mg, Fe)O synthesised at different temperatures and fO(2) conditions show that the Fe3+ conten t can be reliably measured, and that it varies significantly with fO(2 ). Implications of these results to properties of the Earth's interior are discussed.